1. Field of the Invention
The present invention relates to an image sensor and manufacturing method thereof.
2. Related Art
An uncooled (thermal) infrared image sensor (hereinafter, simply “sensor”) is a device including plural pixels having infrared absorbing layers and thermoelectric conversion elements. Each infrared absorbing layer converts an infrared ray into heat, and each thermal conversion element converts this heat into an electric signal.
The uncooled infrared image sensor thermally isolates an infrared absorbing layer and a thermoelectric conversion element of a certain pixel from other pixels and peripheral circuits, to improve sensitivity. In a sensor mounted in a vacuum package, a cavity is provided between a semiconductor substrate and pixels, thereby supporting the pixels above each cavity. With this arrangement, the pixels are thermally isolated from other pixels and the semiconductor substrate. Because the uncooled infrared image sensor does not require a cooler, this sensor has an advantage of compactness and low cost.
When a cavity is attempted to be formed below pixels by using anisotropic etching such as CDE (Chemical Dry Etching), a cavity is also formed below a thin signal wiring located between adjacent pixels. Therefore, the signal wiring is also consequentially thermally isolated from the semiconductor substrate. That is, a portion below a whole pixel region including plural pixels becomes in a cavity state. In a sensor having many pixels such as a QVGA (Quarter Video Graphics Array), this state weakens mechanical strength of the pixel region, thereby generating a problem in reliability.
According to Kosasayama et al. “High sensitive uncooled infrared FPA with SOI diode detectors” ITE TechnicalRepord Vol. 32, No. 6, PP. 21-26, February 2008, an etching stopper film is formed between adjacent pixels by using a DTI (Deep Trench Isolation) process so that a cavity is not formed below the whole pixel region. However, the DTI process requires a fine etching technique, and also has constraints in an etching device and an etching condition, and the like. Therefore, the DTI process cannot be easily performed.